A fuel cell converts chemical energy in fuel directly to electrical energy. A pair of electrodes is installed on either side of an electrolyte membrane, fuel gas containing hydrogen is supplied to the anode and an oxidizing gas containing oxygen is supplied to the cathode. Electrical energy is then extracted using the following electrochemical reaction which takes place on the surfaces of the electrolyte membrane.Anode reaction:H2→2H++2e−  (1)Cathode reaction:2H++2e−+(1/2)O2→H2O  (2)
The fuel gas supplied to the anode can be supplied directly from a hydrogen storage device (e.g., high-pressure tank, liquid hydrogen tank or metal hydride tank), or by supplying hydrogen-containing gas obtained by reforming fuel containing hydrogen (e.g., natural gas, methanol or gasoline). The oxidizing gas supplied to the cathode is generally air.
As shown by equation (2), when the fuel cell is operating, water is produced at the cathode. Also, part of the produced water diffuses from the cathode to the anode, and is discharged from the anode. At this time, if there is a region where too much water vapor is present in the passage supplying the fuel gas, liquid water is produced and flooding will occur. If the gas supply is blocked due to the liquid water, the cell performance decreases and the power generation capacity falls. As shown in equation (2), this water is produced by the reaction, and as the water amount increases as the fuel gas flows from upstream to downstream in the fuel cell, flooding easily occurs in the vicinity of the fuel cell outlet.
JP9-511356A published by the Japanese Patent Office in 1997, discloses a way of preventing flooding by controlling the temperature distribution in the fuel cell. In this prior art, flooding is prevented by making the gas and the coolant flow in the same direction, increasing the temperature from the gas inlet to the outlet, and controlling the temperature distribution in the fuel cell. Due to this, the water produced is incorporated with the gas as steam, and flooding is prevented.